Force

What is Force?

Force refers to any action that, if not countered, will modify how an object moves. This action can involve pushing or pulling objects, leading to changes like speeding up, slowing down, or even bending objects.

For instance, when you push a ball, you’re applying force that causes it to roll. Similarly, if you pull a rope, the force you exert makes the rope move toward you. These examples showcase how force influences the motion or shape of objects.

Forces in Physics

Units and Measurement

In the International System of Units, commonly known as SI, the standard unit used to measure force is Newton (N). This name honors Sir Isaac Newton, a famous scientist who significantly contributed to physics. A newton quantifies how much push or pull is applied to an object.

To understand what a newton represents, consider this explanation. The force of one Newton will move an object that weighs one kilogram at a speed that increases by one meter per second every second. This means if you apply a force of one Newton to a one-kilogram object, it will speed up by one meter per second each second you apply that force.

Vector Nature of Force

Force is not just about how strong it is; it also has a direction. This is why we say force is a vector quantity. A vector quantity has two key features: magnitude and direction. Magnitude means how strong the force is, and direction shows where the force is heading. To visualize this, imagine forces as arrows in drawings. The size of the arrow tells you how strong the force is, and the way the arrow points shows you where the force is going.

Graphically representing forces with arrows helps us understand and analyze the effects of these forces on objects. For instance, if two arrows point in opposite directions, the forces they represent are pulling or pushing against each other. The longer arrow stands for a stronger force. This method allows us to see the combined effect of multiple forces acting on an object, helping us predict the object’s movement and behavior.

Types of Force

Contact Forces

Contact forces come into play when objects touch each other directly. These forces are varied, each with a specific role depending on the nature of the physical contact. For example, the normal force is what you feel when a surface holds an object up against gravity, like a book resting on a table. This force acts perpendicular to the surface, preventing the object from passing through it.

Friction is another type of contact force, providing resistance when objects slide or attempt to slide over each other. It’s what prevents you from slipping as you walk and what makes it possible to grip objects. Tension force occurs in materials that are being stretched, like a rope or cable under pull, maintaining equilibrium and conveying force along its length. Applied force is any force exerted by a person or another object onto a second object, such as pushing a door to open it.

Non-contact Forces

Non-contact forces, or action-at-a-distance forces, operate without the need for physical touch between objects. These forces can act over varying distances, influencing objects far apart from each other. Gravitational force is a prime example, where every mass exerts an attraction on every other mass, like the Earth pulling objects toward its center, affecting everything from falling apples to orbiting planets.

The electromagnetic force plays a critical role in daily life and technological applications, governing interactions between electrically charged particles. This force is behind the electricity and magnetism that power our homes and gadgets. It can attract or repel objects, depending on the nature of their charges, and operates over a range of distances, from atomic scales in atoms and molecules to macroscopic scales in electrical and magnetic fields.

Nuclear forces are another type of non-contact force, which include strong nuclear forces that hold the protons and neutrons together within the nucleus of an atom, despite the repulsive force between like-charged protons. These forces are immensely strong but act over extremely short distances, confined within the atomic nucleus.

Newton's Laws of Motion

Newton's First Law (Law of Inertia)

Newton’s First Law, also known as the Law of Inertia, explains how objects behave in terms of their motion. According to this law, if an object is not moving, it will stay still unless something pushes or pulls it. Similarly, if an object is moving, it will keep moving at the same speed and direction unless something changes its state. This “something” is what we call an external force.

For example, a soccer ball lying on the ground will not start rolling on its own; it stays where it is until a player kicks it. Once kicked, it will continue to roll in a straight line and at a constant speed, assuming no other forces act on it, like friction or air resistance, which can slow it down or alter its path.

Newton's Second Law (Law of Acceleration)

Newton’s Second Law, known as the Law of Acceleration, tells us how the motion of an object changes when forces are applied to it. This law states that the acceleration (the rate at which an object speeds up or slows down) of an object depends on two things: the net force acting on the object and its mass. The net force is the total force considering all the different forces acting on the object, whether they are pushing it or pulling it in any direction.

The law is commonly expressed with the equation $F=ma$ , where $F$ is the net force applied to the object, $m$ is the mass of the object, and $a$ is the acceleration. This equation means that the acceleration of an object increases with more force and decreases with more mass. For example, pushing a car (a heavy object) requires much more force to accelerate it than pushing a bicycle (a lighter object), assuming both need to reach the same acceleration.

Newton's Third Law (Action-Reaction Law)

Newton’s Third Law, often summarized as “For every action, there is an equal and opposite reaction,” highlights the reciprocal nature of forces. This law means that forces always come in pairs: if an object exerts a force on another object, the second object exerts a force that is equal in magnitude but opposite in direction on the first object. This interaction is continuous and occurs regardless of the size or nature of the objects involved.

For example, when you sit on a chair, your body exerts a downward force on the chair due to gravity, and in response, the chair exerts an upward force against your body that is equal in strength. This is why you don’t fall through the chair. Similarly, when a rocket launches, it pushes exhaust gases downwards, and in reaction, these gases exert an upward force on the rocket, propelling it upwards.